The present invention relates to magnetic recording methods. More particularly, the invention relates to a method for rendering visible a latent magnetic image on a recording member which is uniformly magnetized in the direction of movement thereof except for image parts, which have the form of a dot pattern in which the magnetic field is reversed.
In such a magnetic recording method, first a magnetic latent image is formed on a magnetic medium by magnetization. The latent image is then developed with colored magnetic particles, specifically, with a magnetic toner composed of macromolecular resin and magnetic fine particles. The image thus developed is transferred onto a recording sheet using an electrostatic or magnetic method, and then fixed by the application of heat or pressure so as to obtain a permanent copy. Thereafter, residual magnetic toner is removed from the magnetic medium upon which the latent image was formed. The magnetic medium is then ready for use in the next developing cycle. When desired, the magnetic latent image can be erased from the magnetic medium and a new magnetic latent image formed thereon.
There have been proposed a variety of magnetic latent image forming methods for use in magnetic recording. In one of the conventional methods, the magnetizable magnetic medium is made of a thermomagnetic materal such as CrO.sub.2 whose Curie temperature is relatively close to room temperature. In accordance with this method, while a thermal image in the form of a pattern of dots is imposed on the recording medium by means of a thermal head or laser beam, an external magnetic field is applied to the recording medium. A magnetic latent image is thus formed on the recording medium through thermoremanent magnetism by the cooperation of the heat and magnetic field. With this method, the thermomagnetic material is magnetized in advance uniformly in one direction, specifically, the direction of movement of the recording medium, and the external magnetic field is applied in such a manner that the direction of magnetization is reversed by the thermal input section.
For this method, it is possible to use a thermal head having heat generating elements in a number sufficient to provide a desired picture element density, or a laser optical system which inputs a thermal image, in a noncontact manner, with a high density. This method is superior to a magnetic latent image forming method using a magnetic head in that the density of the magnetic latent image which is formed is high. Also, the manufacturing cost for the apparatus is lower.
In a magnetic latent image formed using a thermal head, as shown in FIG. 1, the background is magnetized uniformly in advance in one direction, and the magnetization is reversed, in a dot pattern, only in areas which are heated. Magnetic toner particles adhere to the areas where the field is reversed to produce a visible image.
In this system, unlike an electrostatic latent image developing process, it is unnecessary to charge the toner particles. Therefore, a single component magnetic toner which is stable against environmental changes and which has a long shelf life should be employed as the developing agent.
Methods for developing an electrostatic latent image using a single component magnetic toner are well known. In a device for practicing this method, a latent image is supplied toner from a magnetic "brush", which is a magnetic roll onto which magnetic toner is magnetically attracted, to obtain a visible image. The resultant image is high in quality and the mechanism is simple. However, it is not suitable to use a conventional electrostatic latent image developing device as a magnetic latent image developing device because, in the electrostatic latent image developing method, in order to prevent the background from fogging, a high magnetic field must be applied, several hundred to one thousand gauss or more. However, the coercive force of a magnetic medium employed in the magnetic recording method is on the order of about 1,000 Oe. Accordingly, if a magnetic field sufficiently strong to prevent fogging is applied, the recording medium is demagnetized, that is, the latent image is eradicated, as a result of which the resultant picture is unsatisfactory. Furthermore, repetitive use of the latent image, which is a specific feature of the magnetic copying method, cannot be carried out.
If the magnetic field in the developing section formed by the magnetic roll is sufficiently reduced, the latent image can be prevented from being eradicated. However, if the magnetic force of the magnetic roll is reduced, sufficient magnetic toner cannot be conveyed, as a result of which the latent image is not satisfactorily developed. Furthermore, since the magnetic field of the magnetic roll in the developing section interferes with the magnetic field of the latent image, the magnetic toner tends to stick too firmly to some of the dot areas of the recording medium, and accordingly not transfer completely to the recording sheet. This is another cause of unsatisfactory development.